The present invention relates to a burglar alarm system, and more particularly to such a system designed to detect the breaking of a pane of a sealed multiple pane panel.
U.S. Pat. Nos. 1,974,779 and 3,441,924 disclose pressure-sealed double paned windows with a low pressure or vacuum interior within which a pressure sensor is positioned for detecting a change in pressure, as might result from a window broken in the course of a burglary, and consequently triggering a burglar alarm. While such an alarm system may be useful for pressure-sealed multiple pane panels having a low pressure or vacuum interior or even a high pressure interior, sealed multiple pane panels are presently available with an interior in which the pressure is neither much higher nor much lower than atmospheric, the interior simply being filled with a dry inert gas (such as nitrogen) at approximately atmospheric pressure. Obviously the pressure-based alarm systems described in the aforementioned patents are not useful with such pressure-sealed multiple pane panels and recourse must be had to some non-pressure sensitive alarm system for such panels.
The aforementioned pressure-sensitive burglar alarm systems, and indeed all other burglar alarm systems dependent upon the maintenance of a particular atmospheric parameter within the interior of a sealed multiple pane panel, are incapable of distinguishing between a sharp change in the atmospheric parameter (e.g. pressure) such as might result from a window broken in the course of an attempted burglary, and a gradual change therein, such as might result from aging of the window seals or a structural defect in the window. Thus the prior art burglar alarm systems based on an atmospheric parameter within a sealed interior tend to give a high number of "false" alarms. Not only do such false alarms result in an unnecessary expenditure of energy in mobilizing anti-burglary resources, but, over a period of time, tend to reduce faith in the reliability of the system and result in people ignoring alarms whether they be false or real alarm situations.
Accordingly, it is an object of the present invention to provide a burglar alarm system which in one embodiment is based on an atmospheric parameter other than pressure.
Another object is to provide such a system which--whether pressure-sensitive or not--is in another embodiment capable of distinguishing between low level leakage and the breaking of a pane.
A further object is to provide a burglar alarm system which is simple, inexpensive, easy to maintain and reliable.